This invention relates to inflation elements for use in downhole well tools and, more particularly, to such an element and a method and mold for making the bladder and/or cover portions of such elements where the elastomeric material used to form the bladder and/or cover is processed to impart improved performance characteristics by pre-disposing directional properties via stretching and expansion operations before the tool is run in service.
Downhole inflatable tools such as packers, bridge plugs and the like, have been utilized in subterranean wells for many years. Such tools normally include an inflation element that includes an elastomeric bladder concentrically disposed around a central body portion such as a tube or mandrel. A sheath of axially oriented reinforcing slats or ribs is typically disposed around the bladder. One or more elastomeric cover or seal sections are concentrically disposed around at least a portion of the ribs. Reinforcing structures other than slats and ribs are also common among such tools. Fluid under pressure is introduced from the top of the well or from the interior of the well bore into the central body and through radial passages, or around the exterior body to the interior of the bladder, to cause the bladder and, consequentially, the element to expand. The subject invention is applicable to all known downhole inflatable tools that include a bladder and/or one or more covers.
Typically, the upper ends of the inflatable bladder and ribs are secured relative to the body, while the lower ends of the bladder and reinforcing sheath are secured to a seal which is movable relative to the body. The movable seal responds to inflation forces and allows the inflation element to expand without causing damage to any of its component parts.
For inflation elements of this type, the exposed sections of the reinforcing slats or ribs typically operate as anchor sections, while the elastomeric cover elements typically operate as seals. The anchor section(s) and cover element(s) expand until they engage the wall of the well bore to isolate sections of the well bore on opposite sides of the tool from each other.
Ideally, inflation elements should first expand in the center and then uniformly propagate outwardly in both directions so that fluid is not trapped between the outer surface of the element and the well bore. In addition, the outward expansion should be controlled to prevent relatively steep slopes from occurring in the profile of the bladder during inflation. A steep slope in the profile can cause the bladder to stretch in the axial direction and result in unwanted conditions.
Axial stretching of the bladder during inflation can create two deleterious conditions, 1) localized tri-axial strains in the bladder and 2) pinching seals with related folds in the bladder. Alone, either one of these conditions can cause bladder failure. The presence of both conditions is almost certain to cause failure. Failures occur because the physical properties of the elastomeric material composing the bladder are not adequate to survive service conditions, i.e., highly localized tri-axial strains, high fluid pressure pressing the bladder against the edges of the ribs, elevated temperatures, chemically active (sometimes aggressive) inflation and treatment fluids, etc.
These problems are discussed in an article entitled xe2x80x9cDesign and Testing of a High-Performance Inflatable Packer,xe2x80x9d D. M. Eslinger and H. S. Kohli, SPE Pub. 37483 (1997). FIG. 3 of that article is reproduced as FIG. 3 of the appended drawings to illustrate the pinching and folding problem. Although several solutions were suggested in the article, including the use of specially developed elastomers, slats in the carcass with specific structural features and software to aid in the design and execution of specific jobs, the problems of folds and bladder failure still remain.
Bladder failure was also discussed in my U.S. Pat. No. 5,495,892, which recognized conditions where the bladder tends to pinch and form a seal on the mandrel (central body) during inflation and obstruct the passage of pressurized fluid. The obstruction problem was solved by providing a concentric tube between the outside of the mandrel and the bladder to facilitate fluid communication along the entire length of the bladder regardless of the presence of pinching seals. However, the concentric tube did not eliminate of the formation of folds and the occurrence of other phenomena that cause bladder failures.
Three other patents of mine, U.S. Pat. Nos. 5,469,919, 5,564,504 and 5,813,459, also discuss pinching seals, bladder folding, rib kinking, and rib cutting of the bladder. These patents describe the use of structural elements along the length of the bladder to cause relatively uniform expansion such that the ratio of the largest diameter of the bladder to the smallest diameter during inflation is always below a pre-determined maximum value. However, although the designs in these patents resulted in elements with more uniform expansion, they did not completely eliminate the occurrence of folds, rib kinking, rib cutting or the occurrence of high tri-axial strains in the bladder.
Another problem caused by folds is that they remain when the bladder is deflated. This causes difficulty when the tool is to be retrieved because the bladder cannot deflate to a size that is equal to or smaller than its original run-in size and the tool might not be retrievable. This problem would eliminate the use of relatively low cost thru-tubing inflatable tools from a great many service jobs, and possibly necessitate an expensive xe2x80x9crig jobxe2x80x9d, i.e., such as pulling tubing and requiring other expensive support operations.
In addition, a pinching seal can cause an inflation element to inflate only partially, leaving a significant portion of the element not inflated. At the surface, the tool is thought to be completely inflated and surface operations are continued. However, the pinched seal loses its seal within minutes after inflation operations are terminated. This results in loss of inflation pressure, loss of seal between the inflation element and the well bore, loss of anchor between the inflation element and the well bore and failure of the well tool. This type of failure falls under the general category of a xe2x80x9csoft set failurexe2x80x9d.
The present invention is directed to a method for solving the problems discussed above, a shaping tool or vessel for use in the method, and a downhole tool with a bladder and/or cover sections that are pre-disposed in accordance with the method, where the bladder and/or cover sections have improved inflation characteristics and their elastomeric components have improved physical properties.
Pre-disposing elastomeric components means to better align the long axes of the molecular chains of the elastomer in a direction transverse to the long axis of the tool before the tool is run downhole so that the inflation profiles of the inflation element during inflation are improved and the physical properties of the elastomeric material composing the bladder and/or covers are improved and impart enhanced integrity of the bladder and/or covers in service.
The preferred method of pre-disposing (i.e., stretching) elastomeric components (bladders and covers) is by inflating the components within a shaping tool or vessel having a deliberate interior profile. The interior profile imparts different magnitudes of expansion along the length of the elastomeric component. Pre-disposing of an inflation element in an optimally designed vessel will facilitate improved expansion profiles throughout inflation of the well tool and will abate the formation of pinching seals and folds in the inflation element. Additionally, pre-disposing elastomeric components increases the tear resistance of the elastomer and thereby, increases the tolerance of bladders to rib kinking, rib cutting and folds.
When an elastomeric component such as a bladder or cover section is stretched via inflation, its long molecular chains slide over one another and tend to rotate and translate to become more aligned with the direction of maximum principal strain, i.e., the circumferential direction in the plane normal to the longitudinal axis of the tool. When the component is unloaded (deflated) and allowed to recover, the molecular chains tend to move back toward their original spacial locations and orientations, but do not return entirely back to their original spacial locations. Generally, they return to an intermediate position somewhere between their initial location and the location at the peak of their stretch.
If the elastomeric component is stretched again in the same fashion, the resistance to stretching is less than it was during the first stretching because the molecular chains have already traveled over the same route and their movement is relatively unobstructed. Thus, it can be said that the first stretching produces a pre-disposition in the elastomeric material toward stretching it in the same direction and in the same fashion as had previously been done. This means that the molecular chains in the elastomeric material in their new pre-disposed locations have their long axes more aligned with the direction of the maximum principal strain, which is the direction of stretching.
Thus, when a bladder is inflated in diameter and there is no change in its length, the maximum principal strain is in the circumferential direction in the plane normal to its longitudinal axis. A bladder can therefore be pre-disposed by inflating it in a cylindrical mold. It can then be allowed to deflate and recover to its original size and shape. It can then be installed in an inflation element, or it could be pre-disposed after it is installed in the element. It has been found that when a bladder is pre-disposed in this manner the tear resistance is substantially increased when compared with the tear resistance of a bladder that is not predisposed.
Favorable alignment of the molecular chains are further enhanced if the raw elastomer is processed by calendering it through rollers before it is formed into a bladder or cover. This procedure imparts a strong degree of molecular alignment in the elastomer. This can be been done by forming the elastomer into a thin elongated sheet, wrapping the sheet onto a cardboard mandrel and then slicing the sheet into narrow ribbons approximately 3xe2x80x3-6xe2x80x3 wide. Calendering to acquire a ribbon form to produce a bladder is well known to those skilled in the art. However, calendering to acquire molecular alignment and enhanced physical properties is believed not to be known.
Calendering aligns the long axes of the molecular chains with the long axis of the ribbon before forming it into a bladder. The calendered ribbon is then wrapped onto a mandrel so that the long axis of the ribbon is aligned circumferentially around the bladder or cover, that is, the longitudinal axis of the bladder or cover is transverse to the long axes of the molecular chains.
Pre-disposition of elastomeric components and/or sub-assemblies such as inflation elements can be accomplished by surrounding the component and/or element with a tool having an inner diameter that defines a profiled expansion limit. Fluid is then supplied under pressure for expanding the component and/or element into contact with the inner diameter to impart the pre-disposition. The component is then deflated so it returns generally to its original shape. A component and/or element can be subjected to multiple inflation/deflation cycles.
Because elastomers are inelastic in nature, the component and/or element is kept inflated for a predetermined lapse of time to allow the time-dependent response of the molecular chains to be fully realized. Correspondingly, the magnitude of the stretching, the rate of stretching, the inside contour of the shaping tool, dwell times, the temperature at which the stretching process is performed and the number of stretching cycles performed on a component are all inter-related and can effect the magnitude of pre-disposition and enhancement.
In one embodiment, the shaping tool includes a sleeve that is concentrically positioned to surround the expandable component. The inner diameter of the shaping tool is formed by inserting a pair of structural parts called end caps between the component(s) and sleeve. The end caps define a smooth tapering surface that decreases in diameter from near the center of the tool toward both ends.
Preferably, the inner diameter of the tool has a cylindrical section in the middle, with truncated conical sections at both ends. However, if the expandable component is designed so that it tends to expand initially at the upper end, the truncated conical section at the upper end can be shorter than the one at the lower end, with the cylindrical center section being located closer to the upper end of the shaping tool. The tool can be customized in other ways to impart favorable pre-disposed properties in components having unique physical shapes.
In another embodiment, the cylindrical center section is eliminated, with the inner diameter of the tool being formed of a pair of abutting truncated conical sections with their bases in contact with each other. Other suitable shapes can be used, depending on the pre-disposed profile desired.
This pre-disposition is preferably done at ambient temperature before the downhole tool is run in the well bore.